A demultiplexer (also called a demux) is a device that can route a single input to one of multiple outputs. You can choose what output pin to drive by using the demultiplexer's selection pins. For example, Fig. 1 illustrates how a typical four output demux would operate. Notice how the select pin values relate to which output pin is driven. Driving S1 and S0, to 0 and 1 respectively, selects pin Y1. Driving S1 and S0, to 1 and 0 respectively, selects pin Y2. In binary, '01' equals 1 while '10' equals 2. This pattern continues for all of the other pins. This means if you want to select a certain pin number, you simply need to convert that number to binary. When you send the binary number to the demultiplexer's selection pins, it will select the equivalent output pin. If you are unfamiliar with how to convert decimal numbers to binary numbers, refer to orange tab in the related materials section. If you would like to learn more about binary numbers in general, check out the Marching LEDs project.